There is this old conspiracy theory that claims the amount of balls on a player makes he play better/makes your team play better overall.
If you don't believe it, however, then it doesn't make sense to train keeping. If your player has nothing else to train, you can give him a training boost in a specific attribute instead. (According to the manual => every time a Player is moved to a new zone on the Training Field, he receives a short-term boost in that attribute, which is represented by a small green ball next to his skill balls.)